Environmental barrier coating for a component and method for fabricating the same

ABSTRACT

An environmental barrier coating for a component including silicon that has a first coefficient of thermal expansion is provided. The environmental coating includes a silicon bondcoat that is bonded to at least a portion of an outer surface of the component. An intermediate layer is bonded to the silicon bondcoat and has a second coefficient of thermal expansion matched to the first coefficient of thermal expansion. The intermediate layer has a general composition of RE 2 Si 2 O 7 . A protective layer is bonded to the intermediate layer and has a general composition of RE 2 SiO 5 . A surface layer is bonded to the protective layer. The surface layer includes RE and has a ratio of RE to oxygen of at least 2:3.

BACKGROUND OF THE INVENTION

This invention relates generally to environmental barrier coatings and,more particularly, to environmental barrier coatings for a componentfabricated from a silicon-based substrate material.

Rare earth (RE) disilicate environmental barrier coatings (EBCs), havinga general composition of RE₂Si₂O₇, protect gas turbine componentsfabricated from a silicon-containing ceramic matrix composite (CMC)substrate material or silicon nitride (Si₃N₄) substrate material fromharmful exposure to chemical environments in-service. RE=La, Ce, Pr, Nd,Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, and Lu, and includes the rareearth-like elements Y and Sc. The rare earth disilicates havecoefficients of thermal expansion (CTEs) that are well matched to theCTE of the CMC substrate material. Such disilicates have a minimaltendency to crack in service and during thermal cycling of thecomponent. However, disilicates are vulnerable to leaching of SiO₂ andrecession by chemical interactions with water vapor in the turbinecombustion atmosphere. Such leaching creates a microporousmicrostructure in the EBC, and an initially dense EBC is converted to aporous layer in less than the required design lifetime. Thus, suchdisilicates do not have the durability required for the application.

Rare earth (RE) monosilicates, having a general composition of RE₂SiO₅,have been used as EBCs instead of rare earth disilicates. REmonosilicates have low rates of volatilization in combustion atmospherescontaining water vapor and, hence, have low recession rates. However,the monosilicates typically have CTEs that are not well matched to theCTE of the CMC substrate material. As a result, the monosilicatetopcoats tend to crack during application, heat treatment and/or serviceexposure, allowing water vapor to penetrate the topcoat and causesubsurface chemical reactions and/or premature EBC spallation. Theextent of such cracking is directly dependent on the thickness of thecoating layer and the difference in CTE between the coating layer andthe substrate material.

Many conventional EBC materials are deposited on components using aplasma spraying process. The plasma spraying process providesflexibility to deposit a large variety of materials within a widecoating thickness range (ranging from about 0.002 inch to about 0.040inch) without major process modifications. However, the depositedcoating material is often inherently in a thermodynamically metastablestate (such as an amorphous phase, a higher temperature phase or one ormore non-equilibrium phases) due to rapid quenching during the sprayprocess. Upon exposure to high temperature and transformation to theequilibrium state, the constrained coating can undergo a variety ofdimensional changes resulting in stresses in the coating that can leadto various types of cracking behavior. The propensity of the coating tocrack tends to be directly proportional to the coating thickness.

For a RE₂SiO₅ coating processed by plasma spraying, this is found to beparticularly problematic, leading to both catastrophic through-thicknesscracking and delamination of the coating upon exposure to elevatedtemperatures. In this case, the through-thickness cracking of thecoating material is believed to be mainly driven by the mismatch betweenthe CTE of the coating material, about 6×10⁻⁶¹/C to about 7×10⁻⁶¹/C, andthe CTE of the substrate material, about 4.5×10⁻⁶ ¹/C to about5.0×10⁻⁶¹/C for SiC or a SiC/SiC composite. The delamination of thecoating is observed primarily around non-planar regions of the substratematerial and/or geometrical discontinuities and surface perturbations.Further, the delamination has been attributed to dimensional changesduring the first heating cycle to service temperature. The crackingbehavior has been observed for coatings with a thickness of as low asabout 0.002 inch. Additionally, the coatings processed by plasmaspraying are prone to contain open porosity and/or a network of finecracks intercepting the otherwise closed pores and voids. For EBCapplications, open porosity in the coating can be detrimental. The openporosity provides a path for rapid water vapor penetration and, hence,accelerated localized degradation and/or deterioration of the underlyingmaterials prone to water-vapor mediated oxidation and volatilization.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, the present invention provides an environmental barriercoating for a component including silicon, and having a firstcoefficient of thermal expansion. The environmental barrier coatingincludes a silicon bondcoat that is bonded to at least a portion of anouter surface of the component. At least one intermediate layer isbonded to the silicon bondcoat and has a second coefficient of thermalexpansion matched to the first coefficient of thermal expansion. The atleast one intermediate layer has a general composition of RE₂Si₂O₇. Aprotective layer is bonded to the intermediate layer and has a generalcomposition of RE₂SiO₅. A surface layer is bonded to the protectivelayer. The surface layer includes RE and has a ratio of RE to oxygen ofat least 2:3.

In another aspect, the present invention provides a component for a gasturbine engine. The component includes a substrate material includingsilicon and having a first coefficient of thermal expansion. A siliconbondcoat is bonded to at least a portion of an outer surface of thesubstrate material. At least one intermediate layer is bonded to thesilicon bondcoat. The at least one intermediate layer has a secondcoefficient of thermal expansion matched to the first coefficient ofthermal expansion and has a general composition of RE₂Si₂O₇. Aprotective layer is bonded to the at least one intermediate layer andhas a general composition of RE₂SiO₅. A surface layer is bonded to theprotective layer and includes RE. The surface layer has a ratio of RE tooxygen of at least 2:3.

In another aspect, the present invention provides a method forfabricating a component having an environmental barrier coating. Themethod includes providing the component including silicon having a firstcoefficient of thermal expansion. A bondcoat is bonded to at least aportion of an outer surface of the component. An intermediate layerhaving a general composition of RE₂Si₂O₇ is bonded to the bondcoat. Theintermediate layer has a second coefficient of thermal expansion matchedto the first coefficient of thermal expansion. A protective layer havinga general composition of RE₂SiO₅ is bonded to the intermediate layer. Asurface layer is bonded to the protective layer. The surface layerincludes RE and has a ratio of RE to oxygen of at least 2:3.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional side view of a component coated with anenvironmental barrier coating, according to one embodiment of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide an environmental barriercoating for a silicon-based material, including a ceramic matrixcomposite substrate material or a silicon nitride substrate material,for example. Further, the present invention provides a method forfabricating a component having an environmental barrier coating. In oneembodiment, the environmental barrier coating includes a siliconbondcoat, an intermediate layer having a coefficient of thermalexpansion matched to a coefficient of thermal expansion of theunderlying substrate material, and a thin, uniform recession resistantsurface layer that is crack-resistant during service and durable overthe design life of the underlying component.

Embodiments of the present invention are described below in reference toits application in connection with a component for a gas turbine enginefabricated from a ceramic matrix composite substrate material, a siliconnitride substrate material, a silicon carbide substrate material and/ora metal silicide substrate material. However, it will be obvious tothose skilled in the art and guided by the teachings herein providedthat the invention is likewise applicable to any suitable component thatis fabricated from a silicon-based substrate material. Further, suchcomponents are incorporated into systems including, without limitations,other turbine engines, wherein the components are subjected to extremethermal and/or chemical conditions during system operations. Duringoperation conditions, these components are subjected to combustionenvironments for up to about 32,000 hours at material surfacetemperatures that may exceed 2400° F.

In one embodiment, various components of the gas turbine engine areformed of a ceramic material or ceramic matrix composite (CMC) material.In a particular embodiment, the CMC material is a SiC/SiC CMC material.The SiC/SiC CMC material includes a silicon carbide composite materialinfiltrated with silicon and reinforced with coated silicon carbidefibers. In one embodiment, the ceramic material is a monolithic ceramicmaterial, such as SiC.

Referring to FIG. 1, in one embodiment, an environmental barrier coating10 is applied to a silicon-based material, such as a turbine enginecomponent 15 fabricated from a CMC substrate material 20. Alternatively,turbine engine component 15 is fabricated from a silicon nitride (Si₃N₄)substrate material (not shown). It is apparent to those skilled in theart and guided by the teachings herein provided that turbine enginecomponent 15 can be fabricated from any suitable silicon-based material.

Environmental barrier coating 10 includes at least one intermediatelayer 25 bonded to or deposited on CMC substrate material 20 and bondedthereto. As used herein, references to the term “bonded” are to beunderstood to include direct and indirect bonding through another layer,such as a bondcoat or an intermediate layer. In one embodiment,intermediate layer 25 has a total thickness of about 3.0 mil to about10.0 mil. Intermediate layer 25 has a coefficient of thermal expansionthat is matched to the coefficient of thermal expansion of substratematerial 20. As used herein, references to the term “matched,” inreference to a coefficient of thermal expansion, are to be understood torefer to a first layer, such as intermediate layer 25, having acoefficient of thermal expansion within about +/−15% of a coefficient ofthermal expansion of a second layer, such as substrate material 20. Inone embodiment, intermediate layer 25 has a coefficient of thermalexpansion similar to the coefficient of thermal expansion of CMCsubstrate material 20. In a particular embodiment, intermediate layer 25has a coefficient of thermal expansion substantially equal to thecoefficient of thermal expansion for CMC substrate material 20. As shownin FIG. 1, in one embodiment, environmental barrier coating 10 includesa silicon bondcoat 30 applied to or deposited on CMC substrate material20 before intermediate layer 25 is applied to or deposited on CMCsubstrate material 20. In a particular embodiment, bondcoat 30 has acoefficient of thermal expansion matched to substrate material 20 and/orintermediate layer 25.

In one embodiment, intermediate layer 25 includes at least one layerformed from at least one rare earth (RE) silicate including, but notlimited to, Y, Dy, Ho, Er, Tm, Tb, Yb and/or Lu, having a generalcomposition of RE₂Si₂O₇. For example, a first intermediate layer 27includes RE₂Si₂O₇. Additionally, intermediate layer 25 includes a secondor outer intermediate layer 29 positioned between first intermediatelayer 27 and an external barrier layer, such as a recession resistantsurface layer 35, as shown in FIG. 1. Although only two intermediatelayers 27, 29 are shown in FIG. 1, it is apparent to those skilled inthe art and guided by the teachings herein provided that intermediatelayer 25 can include any suitable number of layers. In one embodiment,the layers have a combined thickness of about 3.0 mil to about 10.0 mil.In addition to the at least one rare earth silicate, or alternatively,intermediate layer 25 may include at least one suitable material layerand/or each intermediate layer 27, 29 may include at least one suitablematerial.

Recession resistant surface layer 35 is applied to or deposited onintermediate layer 25. In one embodiment, recession resistant surfacelayer 35 is applied to or deposited on intermediate layer 25 using aphysical vapor deposition or chemical vapor deposition process.Alternatively, recession resistant surface layer 35 is applied tointermediate layer 25 using infiltration by an aqueous solution of asalt of the desired compound, which is then decomposed to yield thedesired oxide or metal in the micropores. Recession resistant surfacelayer 35 and/or the infiltrated layer can be applied using any suitableprocess known to those skilled in the art and guided by the teachingsherein provided including, but not limited to, sol gel technology,plasma spray, slurry spray and/or paint spray processes.

In one embodiment, recession resistant surface layer 35 is applieduniformly to or deposited uniformly on outer intermediate layer 29. Inthis embodiment, recession resistant surface layer 35 has a thickness ofabout 0.5 mil to about 2.0 mil. Recession resistant surface layer 35 hasa suitable thickness to prevent or resist cracking, penetration by watervapor and/or peeling in-service. In alternative embodiments therecession resistant surface layer 35 has a suitable thickness thatprevents or resists cracking, water vapor penetration and/or peeling.For example, in one embodiment, recession resistant layer has athickness less than about 0.001 inch (25 micrometers).

In one embodiment, recession resistant protective layer 36 includes atleast one rare earth (RE) silicate including, but not limited to, Y, Dy,Ho, Er, Tm, Th, Yb and/or Lu, having a general composition of RE₂SiO₅.Recession resistant protective layer 36 is chemically compatible withthe underlying intermediate layer 25 and has a high recessionresistance. For example, recession resistant protective layer 36 mayinclude Y₂SiO₅ applied to or deposited on intermediate layer 25containing a Y₂Si₂O₇ outer surface. Surface layer 38 is chemicallycompatible with the underlying protective layer 36 and has highrecession resistance. Surface layer 38 may include Y₂O₃ applied to ordeposited on protective layer 36. It is apparent to those skilled in theart and guided by the teachings herein provided that recession resistantprotective layer 36 and recession resistance surface layer 38 caninclude any suitable material including one or more rare earth metals.

In one embodiment, recession resistant surface layer 35 includes aprotective layer 36 bonded to intermediate layer 25. In this embodiment,protective layer 36 has a general composition of RE₂SiO₅. Recessionresistant surface layer 35 further includes a surface layer 38 bonded toprotective layer 36. Surface layer 38 includes at least one RE, namely,Y, Dy, Ho, Er, Tm, Th, Yb and/or Lu, and has a ratio of RE to oxygen ofat least 2:3. Surface layer 38 includes RE₂O₃, a RE metal, a saltcomprising RE and/or an organometallic material including RE. Suitableorganometallic material includes, without limitation, a suitablealcoxide including a metallic specie including a RE, such as, withoutlimitation, RE-isopropoxide (RE-[OCH(CH₃)₂]₃), RE-tri-n-butoxide(RE-(OC₄H₉)₃), or RE-ethoxide (RE-(OC₂H₅)₃). In a particular embodiment,surface layer 38 has a general composition of RE₂O₃, or reacts with agas phase to from the general composition of RE₂O₃.

In one embodiment, protective layer 36 is formed by a chemical reactionof surface layer 38 with intermediate layer 25. In a particularembodiment, an interface between bondcoat 30 and intermediate layer 25,and an interface between protective layer 36 and surface layer 38 issubstantially continuous. Alternatively, protective layer 36 isdeposited directly onto intermediate layer 25 and surface layer 38 isdeposited directly onto protective layer 36, substantially withoutinterlayer chemical reactions, to form a multilayer environmentalbarrier coating 10. In this embodiment, the interfaces between bondcoat30 and intermediate layer 25, intermediate layer 25 and protective layer36, and protective layer 36 and surface layer 38 may also besubstantially continuous.

In one embodiment, intermediate layer 25 includes a microvoided surfaceregion infiltrated with a recession resistant oxide to form, protectivelayer 36 on a surface region forming the microvoids. Themicrovoided-surface region or microstructure may be created by allowinga surface layer to react with water vapor at a sufficiently hightemperature for a selected time duration to create a microvoidedmicrostructure. Alternatively, the microvoided or porous microstructuremay be created by plasma spraying. For example, a layer of microvoidedRE₂SiO₅ can be created on intermediate layer 25 including RE₂Si₂O₇ byexposure to a flowing water vapor rich atmosphere for about 100 hours toabout 500 hours at about 2400° F. The microvoids are then infiltratedwith a chemically compatible, water vapor resistant oxide to createrecession resistant surface layer 35. For example, the microvoidedRE₂SiO₅ created on the RE₂Si₂O₇ intermediate layer 25 can be infiltratedwith RE₂O₃ to produce a recession resistant surface layer 35(RE₂O₃+RE₂SiO₅) that protects the underlying RE₂Si₂O₇ intermediate layer25, silicon bondcoat 30 and CMC substrate material 25 from water vapordamage.

In a particular embodiment, intermediate layer 25 includes Y₂Si₂O₇ andprotective layer 36 includes a microvoided Y₂SiO₅ layer infiltrated withY₂O₃ to form recession resistant surface layer 35. Although therecession resistant oxide may not have a CTE that is well matched to theCTE of CMC substrate material 20 and/or the CTE of intermediate layer25, recession resistant surface layer 35 is thin enough to prevent orresist cracking and water vapor penetration. Modeling indicates that a0.001 inch thick Y₂SiO₅ recession resistant surface layer having a CTEof about 6.5×10⁻⁶¹/C and elastic modulus E=30-40 GPa, applied to a CMCsubstrate material with a Y₂Si₂O₇ intermediate layer 25 having a CTE ofabout 4.5×10⁻⁶¹/C and elastic modulus E=280 GPa, should fall below thecracking limit based on a strain-energy-release-rate criterion forthrough-thickness cracking.

In an alternative embodiment, recession resistant surface layer 35includes a microvoided surface region infiltrated with a metal-oxideprecursor of a recession resistant oxide. In one embodiment, amicrovoided surface region, as described above, is infiltrated with aselected concentration of metal, which is then oxidized to formrecession resistant surface layer 35. The infiltrant can be an aqueousor nonaqueous solution of a soluble RE salt, a suspension of submicronRE-containing particles or a solution that produces a metallic RE upondrying. Subsequent reaction or oxidation of the infiltrated RE fills themicrovoids and produces recession resistant protective layer 35 having ageneral composition of RE₂SiO₅ on pore surfaces and/or the externalsurface.

In a particular embodiment, intermediate layer 25 includes Y₂Si₂O₇ andprotective layer 36 includes a microvoided Y₂SiO₅ layer infiltrated withY to form a dense, recession resistant surface layer 35 including Y₂SiO₅and Y₂O₃. During this process, the volume increase upon oxidationfacilitates filling of the micropores to create a dense surfacemicrostructure. The dense, recession resistant surface layer 35 bondedto the thicker intermediate layer 25, e.g. Y₂Si₂O₇, produces a surfaceregion that is highly resistant to water vapor penetration and is toothin to crack as a result of a CTE mismatch with substrate material 20.Additionally, the thicker intermediate layer 25 provides furtherprotection to substrate material 20 should recession resistant surfacelayer 35 be breached, as well as a convenient substrate for surfacetailoring.

Intermediate layer 25 having a general composition of RE₂Si₂O₇ initiallymay have inadequate recession resistance in a combustion atmospherecontaining water vapor. In one embodiment, intermediate layer 25 iscoated with a thin layer having a general composition of RE₂O₃. Thelayers react chemically upon exposure to elevated temperatures to formin-situ a thin, dense protective layer 36 having a general compositionof RE₂SiO₅, which is chemically compatible with RE₂Si₂O₇. Protectivelayer 36 and, thus, recession resistant surface layer 35 have highrecession resistance and a high cracking resistance. In a particularembodiment, a thin, uniform surface layer of a recession resistantoxide, such as RE₂O₃, is applied to intermediate layer 25 includingRE₂Si₂O₇ using at least one of a physical vapor deposition, chemicalvapor deposition, sol-gel coating, slurry coating, plasma spraying orpaint spraying process. The RE₂O₃ layer reacts with the underlyingRE₂Si₂O₇ layer to form a thin RE₂SiO₅ layer. Although the RE₂SiO₅ layeris not well CTE matched to the underlying RE₂Si₂O₇ layer, cracking isprevented or limited because the RE₂SiO₅ layer is thin, e.g., less thanabout 0.001 inch (25 micrometers), and the volume change of the reactionRE₂O₃+RE₂Si₂O₇=2RE₂SiO₅ is only about 1%. Further, the RE₂O₃/RE₂Si₂O₇reaction can be beneficial in eliminating or reducing surface cracks andpores and producing a microstructurally favorable surface layer.

In one embodiment, the initial RE₂Si₂O₇ layer is infiltrated by aprecursor solution including at least one RE, such as an aqueoussolution including an inorganic RE salt, a non-aqueous solutionincluding a RE-alcoxide or a carrier for metallic RE, prior to or inlieu of deposition of the RE₂O₃ layer. Penetration of the RE-includingprecursor into open pores in the RE₂Si₂O₇ layer results in formation ofRE₂SiO₅ on pore walls when exposed to an elevated temperature. Suchin-situ formed RE₂SiO₅ provides further protection of the porousRE₂Si₂O₇ substrate from exposure to water vapor.

In one embodiment, the present invention provides a method forfabricating a component, such as gas turbine engine component 15, havingenvironmental barrier coating 10. Gas turbine engine component 15includes silicon and has a first coefficient of thermal expansion.Intermediate layer 25 is bonded to an outer surface of gas turbineengine component 15. Intermediate layer 25 has a second coefficient ofthermal expansion within about +/−15% of the coefficient of thermalexpansion of gas turbine engine component 15. In a particularembodiment, a second intermediate layer is bonded to the firstintermediate layer. In this embodiment, the second intermediate layerhas a coefficient of thermal expansion similar or identical to thecoefficient of thermal expansion of the first intermediate layer.Alternatively, the second intermediate layer has a coefficient ofthermal expansion different than the coefficient of thermal expansion ofthe first intermediate layer. It is apparent to those skilled in the artand guided by the teachings herein provided that environmental barriercoating 10 can include any suitable number of intermediate layers.

Protective layer 36, having a general composition of RE₂SiO₅, is bondedto the outer intermediate layer. In one embodiment, protective layer 36is deposited on intermediate layer 25. In a particular embodiment,protective layer 36 is bonded to intermediate layer 25 by a reaction ofa metal-oxide precursor material with intermediate layer 25. Forexample, protective layer 36 including RE₂SiO₅ is formed by reacting ametal-oxide precursor material including RE₂O₃ with intermediate layer25 including RE₂Si₂O₇. In one embodiment, surface layer 38 is bonded toprotective layer 36. Surface layer 38 includes at least one RE and has aratio of RE to oxygen of at least 2:3.

Alternatively, protective layer 36 is formed and bonded to intermediatelayer 25 by heat treating environmental barrier coating 10 after surfacelayer 38 is deposited on intermediate layer 25. In another alternativeembodiment, protective layer 36 is bonded to intermediate layer 25 byforming protective layer 36 in-situ during service of the environmentalbarrier coated CMC component.

The above-described environmental barrier coating for a silicon-basedcomponent, e.g., a SiC/SiC CMC or silicon nitride material component, iscrack resistant due to the CTE compatibility between the substratematerial and the intermediate layer, and the dense, highly recessionresistant surface layer that is too thin to exhibit deleteriouscracking. The recession resistant surface layer prevents or limitsexposure of the underlying layers to the combustion atmosphere. Theenvironmental barrier coating has adequate durability and recessionresistance to meet the design lifetime goal of up to about 32,000service hours at a temperature of about 2400° F. Further, theintermediate layer provides further protection to the underlyingsubstrate material should the recession resistant surface layer bebreached, as well as a convenient substrate for surface tailoring.

Exemplary embodiments of an environmental barrier coating and a methodfor fabricating components including the environmental barrier coatingare described above in detail. The environmental barrier coating andmethod are not limited to the specific embodiments described herein, butrather, components of the environmental barrier coating and/or steps ofthe method may be utilized independently and separately from othercomponents and/or steps described herein. Further, the describedcomponents and/or method steps can also be defined in, or used incombination with, other coatings and/or methods, and are not limited topractice with only the environmental barrier coating and method asdescribed herein.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. An environmental barrier coating for a component comprising siliconhaving a first coefficient of thermal expansion, said environmentalcoating comprising: a silicon bondcoat bonded to at least a portion ofan outer surface of said component; at least one intermediate layerbonded to said silicon bondcoat and having a second coefficient ofthermal expansion matched to said first coefficient of thermalexpansion, said intermediate layer having a general composition ofRE₂Si₂O₇; a protective layer bonded to said intermediate layer andhaving a general composition of RE2SiO5; and a surface layer bonded tosaid protective layer and comprising RE and having a ratio of RE tooxygen of at least 2:3.
 2. An environmental barrier coating inaccordance with claim 1 wherein said protective layer is formed by achemical reaction of said surface layer with said at least oneintermediate layer, and each interface between each of said bondcoat,said at least one intermediate layer, said protective layer and saidsurface layer is substantially continuous.
 3. An environmental barriercoating in accordance with claim 1 wherein said surface layer has ageneral composition of RE₂O₃, or reacts with a gas phase to from thegeneral composition of RE₂O₃, wherein RE comprises an element selectedfrom the group consisting of Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
 4. Anenvironmental barrier coating in accordance with claim 1 wherein saidprotective layer is deposited directly onto said at least oneintermediate layer and said surface layer is deposited directly ontosaid protective layer, substantially without interlayer chemicalreactions, to form a multilayer environmental barrier coating, andinterfaces between said bondcoat, said at least one intermediate layer,said protective layer and said surface layer are substantiallycontinuous.
 5. An environmental barrier coating for a componentcomprising silicon having a first coefficient of thermal expansion, saidenvironmental coating comprising: a silicon bondcoat bonded to at leasta portion of an outer surface of said component; an inner intermediatelayer bonded to said silicon bondcoat, said inner intermediate layerhaving a second coefficient of thermal expansion matched to said firstcoefficient of thermal expansion, said inner intermediate layer having ageneral composition of RE₂Si₂O₇; an outer intermediate layer bonded tosaid inner intermediate layer and forming a plurality of microvoids; aprotective layer bonded to said intermediate layer and having a generalcomposition of RE₂SiO₅; and a surface layer bonded to said protectivelayer and comprising RE and having a ratio of RE to oxygen of at least2:3, wherein said outer intermediate layer is infiltrated with saidsurface layer to form said protective layer on a surface forming eachmicrovoid of said plurality of microvoids.
 6. An environmental barriercoating in accordance with claim 1 wherein said second coefficient ofthermal expansion is within +/−15% of said first coefficient of thermalexpansion.
 7. An environmental barrier coating in accordance with claim1 wherein said surface layer comprises a material selected from thegroup consisting of RE₂O₃, a RE metal, a salt comprising RE, and anorganometallic material comprising RE.
 8. An environmental barriercoating in accordance with claim 1 wherein said at least oneintermediate layer has a thickness of about 3.0 mil to about 10.0 mil.9. An environmental barrier coating in accordance with claim 1 whereinsaid surface layer has a thickness of about 0.4 mil to about 2.0 mil.10. An environmental barrier coating in accordance with claim 1 whereinsaid component comprises a material selected from the group consistingof a ceramic matrix composite material, a silicon nitride material, asilicon carbide material, and a metal silicide material.
 11. Anenvironmental barrier coating in accordance with claim 1 wherein saidcomponent comprises a SiC—SiC ceramic matrix composite substratematerial.
 12. An environmental barrier coating in accordance with claim1 wherein at least one of said at least one intermediate layer, saidprotective layer and said surface layer comprises at least one anelement selected from the group consisting of Y, Dy, Ho, Er, Tm, Tb, Yb,and Lu.
 13. A component for a gas turbine engine, said componentcomprising: a substrate material comprising silicon, said substratematerial having a first coefficient of thermal expansion; a siliconbondcoat bonded to at least a portion of an outer surface of saidsubstrate material; at least one intermediate layer bonded to saidsilicon bondcoat and having a second coefficient of thermal expansionmatched to said first coefficient of thermal expansion, the intermediatelayer having a general composition of RE₂Si₂O₇; a protective layerbonded to said intermediate layer and having a general composition ofRE₂SiO₅; and a surface layer bonded to said protective layer andcomprising RE and having a ratio of RE to oxygen of at least 2:3.
 14. Acomponent in accordance with claim 13 wherein at least one of said atleast one intermediate layer, said protective layer and said surfacelayer comprises an element selected from the group consisting of Y, Dy,Ho, Er, Tm, Tb, Yb, and Lu.
 15. A component in accordance with claim 13wherein said second coefficient of thermal expansion is within +/−15% ofsaid first coefficient of thermal expansion.
 16. A method forfabricating a component having an environmental barrier coating, saidmethod comprising: providing the component comprising silicon having afirst coefficient of thermal expansion; bonding to a bondcoat to atleast a portion of an outer surface of the component; bonding anintermediate layer having a second coefficient of thermal expansionmatched to the first coefficient of thermal expansion to the bondcoat,said intermediate layer having a general composition of R E₂Si₂O₇;bonding a protective layer having a general composition of RE₂SiO₅ tothe intermediate layer; and bonding a surface layer comprising RE andhaving a ratio of RE to oxygen of at least 2:3 to the protective layer.17. A method in accordance with claim 16 wherein bonding a protectivelayer to the intermediate layer further comprises reacting the surfacelayer with the intermediate layer to form the protective layer.
 18. Amethod in accordance with claim 16 wherein bonding a protective layer tothe intermediate layer further comprises: forming a plurality ofmicrovoids on a surface region of the intermediate layer; andinfiltrating the plurality of microvoids with the surface layer to formthe protective layer within the plurality of microvoids.
 19. A method inaccordance with claim 16 wherein bonding a protective layer to theintermediate layer further comprises forming the protective layer byheat treating the environmental barrier coating.
 20. A method inaccordance with claim 16 wherein bonding a protective layer to theintermediate layer further comprises forming the protective layerin-situ during service of the gas turbine engine component.
 21. Anenvironmental barrier coating for a component comprising silicon havinga first coefficient of thermal expansion, said environmental coatingcomprising: a silicon bondcoat bonded to at least a portion of an outersurface of said component; an inner intermediate layer bonded to saidsilicon bondcoat, said inner intermediate layer having a secondcoefficient of thermal expansion matched to said first coefficient ofthermal expansion, said inner intermediate layer having a generalcomposition of RE₂Si₂O₇; an outer intermediate layer bonded to saidinner intermediate layer and forming a plurality of microvoids; aprotective layer bonded to each microvoid, the protective layer having ageneral composition of RE₂SiO₅; and a surface layer bonded to saidprotective layer within each microvoid, the surface layer comprising REand having a ratio of RE to oxygen of at least 2:3.